Method for resource management in a cellular communication network and resource management system

ABSTRACT

A method for resource management in a cellular communication network, wherein the network includes at least one base station per cell for a plurality of cells that operates in TDD (Time Division Duplex) transmission mode, wherein the base stations implement a respective UL/DL configuration mode in which frames are composed of a specific sequence of downlink DL, uplink UL and special S subframes, is characterized in that the base stations are operated such that neighboring base stations with adjacent and/or overlapping coverage areas employ different UL/DL configuration modes, wherein interference between neighboring base stations is taken account of by implementing power control on a subframe basis. Furthermore, a resource management system for use in a cellular communication network is described.

The present invention relates to a method for resource management in acellular communication network, wherein said network includes at leastone base station per cell for a plurality of cells that operates in TDD(Time Division Duplex) transmission mode, wherein said base stationsimplement a respective UL/DL configuration mode in which frames arecomposed of a specific sequence of downlink DL, uplink UL and special Ssubframes.

Furthermore, the present invention relates to a resource managementsystem for use in a cellular communication network, wherein said networkincludes at least one base station per cell for a plurality of cellsthat operates in TDD (Time Division Duplex) transmission mode, whereinsaid base stations are configured to implement a respective UL/DLconfiguration mode in which frames are composed of a specific sequenceof downlink DL, uplink UL and special S subframes.

Time Division Duplex (TDD) is a transmission mode supported both in 3GPPUMTS, LTE (Long Term Evolution) and IEEE 802.16 that utilizes the sameradio access scheme as the Frequency Division Duplex (FDD), i.e. in caseof LTE OFDMA (Orthogonal Frequency-Division Multiple Access) in thedownlink and the SC-FDMA (Single Carrier Frequency-Division MultipleAccess) in the uplink, CDMA in case of UMTS and OFDMA in case of IEEE802.16 in both uplink and downlink. Furthermore, TDD uses the samesubframe format as well as the same configuration protocols as FDD. Themain difference compared with FDD is that TDD macro cellular basestations, or evolved Node B (eNBs) in 3GPP terminology, support anunpaired frequency band, where downlink and uplink are separated in timedomain, with each frame being composed by downlink (DL), uplink (UL) andspecial (S) sub-frames.

Special sub-frames are used to switch from downlink to uplink and theyare included at least once within each frame. In particular, the specialsub-frame consists of the following three special fields, a downlinkpart (DwPTS), a guard period (GP), and an uplink part (UpPTS). In 3GPPLTE, the UL/DL portion of each frame may be configured according to thespecification provided in document 3GPP TS 36.300, TechnicalSpecification Group Radio Access Network; Evolved Universal TerrestrialRadio Access (E-UTRA) and Evolved Universal Terrestrial Radio AccessNetwork (E-UTRAN); Overall description; Stage 2 (Rel 10), April 2011,which defines 7 different UL/DL configuration modes as shown in thefollowing table:

Uplink-downlink allocations Switch- Config- point Subframe numberuration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms DL S UL UL UL DL S UL ULUL 1 5 ms DL S UL UL DL DL S UL UL DL 2 5 ms DL S UL DL DL DL S UL DL DL3 10 ms  DL S UL UL UL DL DL DL DL DL 4 10 ms  DL S UL UL DL DL DL DL DLDL 5 10 ms  DL S UL DL DL DL DL DL DL DL 6 5 ms DL S UL UL UL DL S UL ULDL

Despite the provision of flexibility in resource allocation, suchasymmetric UL/DL brings new challenges in admission control and loadbalancing and introduces certain limitations due to interferencereasons. In the case that two adjacent eNBs apply different UL/DLconfiguration models, it may happen that eNBs directly interfere eachother, or UEs (User Equipment) are receiving direct interference fromeach other as illustrated in FIG. 1, where eNB A, serving User EquipmentUE 1, is transmitting in DL towards UE 1, while eNB B, serving UserEquipment UE 2, is receiving in UL from UE 2. Such a scenario results incross-slot interference including eNB-to-eNB interference, which mayprove significantly severe when a line of sight exists among eNBs. ForUEs in close proximity, UE-to-UE interference is severe and difficult tohandle also due to mobility. Due to physical limitations of the RF(Radio Frequency) frontend at eNBs and UEs, there is the need to avoidsuch cross-interference.

In particular, although UL/DL configuration may be configured accordingto traffic demand and changed dynamically to reflect traffic variationsby altering the UL/DL frame portion, conventional interference controlmethods require neighbor eNBs to be synchronized following the sameUL/DL configuration mode, which is a major barrier to flexibility. Itcan thus be noted that there is generally a trade-off betweenflexibility and interference, as also acknowledged in document Peter W.C. Chan, et. al., “The Evolution Path of 4G Networks: FDD or TDD?”, IEEECommunication Magazine, Vol. 44, No. 12, December 2006. However, thedynamic UL/DL configuration mode is a desired feature of TD-LTE, asdiscussed in the document cited above as well as in document D. Astely,E. Dahlman, A. Furuskär, S. Parkvall, “TD-LTE—The radio-access solutionfor IMT-Advanced/TDD”, CHINACOM, August 2010.

Previous efforts to provide cell-independent asymmetric UL/DLconfiguration, in which neighboring cells adopt a different UL/DLconfiguration, concentrate on evaluating loose UL/DL synchronizationamong neighboring cells like Adaptive TDD (as described, e.g., in G.Szladek, B. Heder, J. Bito, “Investigation of Interference conditions inBFWA System Applying Adaptive TDD”, IEC, Prague, 2005) and furthermethods that utilize opportunistic interference mitigation (asdescribed, e.g., in E. Foutekova, P. Agyapong, H. Haas, “ChannelAsymmetry and Random Time Slot Hopping in OFDMA-TDD Cellular Networks”,IEEE VTC Spring, Singapore, May 2008). Although such methods provideflexibility, their performance is subject to the degree of correlationof UL/DL configuration modes among neighboring cells as well as on theassociated traffic load. Thus their use suit best cases where the UL/DLmode is similar with small sub-frame differences, providing only limiteddegree of flexibility.

An alternative method is the Hybrid Division Duplex (HDD), whichcombines both FDD and TDD schemes within each cell (described, e.g., inS. Yun, et.al., “Hybrid Division Duplex System for Next-GenerationCellular Services”, IEEE Transactions on Vehicular Technology, Vol. 56,No. 5, September 2007) splitting each cell into two regions, with theouter macro cell region operating in FDD mode and the inner in TDD. HDDmay resolve cross-slot interference for UEs located at the edge of eachcell due to the use of the FDD, while being capable to providecell-independent asymmetric UL/DL configurations among adjacent cellsvia the TDD mode. Such a proposal provides a high degree of UL/DLconfiguration mode flexibility, but it is subject to the use of FDDmode, which might not be available for certain deployment scenarios thatrely solely on TDD.

Furthermore, US 2009/0249153 A1 discloses a dynamic adjustment of theUL/DL configuration mode with the purpose of introducing alternations ofselected sub-frames for UL to DL or vice versa. The means of sub-frameadjustment is the introduction of a new sub-frame referred to as“mute”-sub-frame, where the eNB is neither in DL nor UL mode. Such asub-frame is used to provide synchronization among neighboringcells/eNBs and/or associated UEs in order to introduce a sub-framechange, i.e. a dynamic UL/DL configuration adjustment, smoothly withoutinterruption and performance degradation avoiding cross-slotinterference. Such a method provides the means of concurrent dynamicadjustment of the UL/DL configuration mode among a set of neighboringcells or for an individual cell but is not addressing the problem ofproviding a different UL/DL configuration mode among neighboring cellsor different network regions.

It is therefore an object of the present invention to improve andfurther develop a method and a system of the initially described typefor resource management in a cellular communication network in such away that a high degree of UL/DL configuration mode flexibility isprovided without introducing significant cross-interference.

In accordance with the invention, the aforementioned object isaccomplished by a method comprising the features of claim 1. Accordingto this claim, such a method is characterized in that said base stationsare operated such that neighboring base stations with adjacent and/oroverlapping coverage areas employ different UL/DL configuration modes,wherein interference between neighboring base stations is taken accountof by implementing power control on a subframe basis.

Furthermore, the aforementioned object is accomplished by a systemcomprising the features of claim 23. According to this claim, such asystem is characterized in that it comprises a management mechanism thatcontrols the operation of base stations in such a way that neighboringbase stations with adjacent and/or overlapping coverage areas employdifferent UL/DL configuration modes, wherein interference betweenneighboring base stations is taken into account by implementing powercontrol on a subframe basis.

According to the invention it has first been recognized that the abovementioned trade-off between flexibility and interference can be resolvedby allowing different individual base stations (or network regions, i.e.group of base stations) to operate at the same time in different accessmodes, i.e. UL or DL, while potential interference (or other performancedegradations) is repressed by the way of controlling the transmissionpower for individual subframes. Specifically, a management mechanism isprovided that controls the operation of neighboring base stations insuch a way that they may be assigned different UL/DL configurationmodes, while subframe power control allows for interference-decouplingbetween those base stations with different patterns. Thus, the methodaccording to the invention reduces or even avoids cross-interferencebetween base stations and UEs. Power control may also apply UL/DLpatterns including phases with neither UL nor DL such thatcross-interference is avoided.

The method according to the invention provides a higher degree offlexibility compared to adaptive TDD and to opportunistic interferencemitigation, relies only on TDD deployment and its objective is notlimited to adjusting the UL/DL configuration but to configuredynamically different UL/DL modes into neighboring cells and regions.Thus, the main advantage of this invention is that it allows fordifferent UL/DL configurations among neighbor cells in TD-LTE enhancingthe network performance. In this way the network resources are exploitedmore efficiently, e.g. by matching the traffic/resource demand with theUL/DL configuration mode, thereby capturing diverse and evolving trafficdemands. Such solution should follow the Self-Organized Network (SON)paradigm, as described in document 3GPP TR 36.902, Evolved UniversalTerrestrial Radio Access Network (E-UTRAN); Self-configuring andself-optimizing network (SON) use cases and solutions, April 2011, beinga part of the eNB self-configuration process upon a new eNB installationand may also re-configure an existing UL/DL mode based on dynamictraffic conditions.

According to a preferred embodiment base stations may be configured toidentify those subframes which are potentially subject to interference.Hereinafter, such subframes will be briefly denoted “potentiallyinterfering subframes”. Essentially, cross-interference will occur insituations in which a base station has two neighboring base stationswith adjacent and/or overlapping coverage areas that employ a differentUL/DL configuration mode. For instance, one of the neighboring basestations may operate in UL, while in the same subframe another of theneighboring base stations operates in DL. In such case, a base stationwould identify its respective subframe as potentially interferingsubframe.

Advantageously, once potentially interfering subframes have beenidentified, a base station may perform power control for thesesubframes. In a specific embodiment it may be provided that a basestation reduces its transmission power for such subframes, eitherdynamically or by a predefined amount. In certain situations, e.g. incases of high traffic load, potentially interfering subframes can bemade completely “silent”, which means that the base station does not atall transmit or receive in such subframes. In such case the respectivebase station functions as a kind of “buffer” base station between twoneighboring base stations that operate in different UL/DL configurationmodes.

According to a further preferred embodiment a base station, in case apotentially interfering subframe is a special S subframe, keeps usingthe DL part of said S subframe, provided a neighboring base stationswith adjacent and/or overlapping coverage area also employs a DLsubframe. The reason is the fact that a DL part of a special S frame islike a conventional DL subframe, while the same does not hold for the ULpart since this contains additional synchronization information.

With respect to an efficient implementation of power control it may beprovided that subframes with a fixed maximum power are introduced. Thefixed maximum power may be specified on the basis of a precedingtraining phase. According to another embodiment power control may beimplemented by introducing time-zones with different maximum power,wherein UEs are scheduled to these time-zones depending on theirlong-term Signal-to-Noise Ratio SNR (Signal-to-Noise Ratio). Again, arespective preceding training phase for specifying maximum power may beperformed.

Additionally or alternatively, power control may include performingper-frame signaling among potentially interfering base stations andadjusting power sequentially for the purpose of interferencecancellation.

According to another embodiment power control may include controllingthe maximum power depending on the backhaul capacity and/or the numberof UEs being served by the involved base stations. According to stillanother embodiment power control may include excluding parts of thespectrum in the DL. Sparing out parts of the spectrum in the DL reducesalso the emitted power in time-domain and avoids the problem that theuseful signal is either “drowned” in noise or the automatic gain-controlreaches the saturation region, i.e. FR>1 at one base station to avoiddrowning a UE at the other base station in the UL.

Beside the regulation of sub-frame transmission, the introduction of adifferent UL/DL configuration mode among certain neighbor base stationsrequires also a process of selecting the optimal UL/DL configurationmode for each base station. In a specific embodiment the UL/DLconfiguration mode selection may be performed cooperatively among basestations based on information provided by a central network managemententity and/or by the base stations themselves. In particular, thisinformation may include details about an expected resource demand, whichmay be determined for each base station on the basis of an estimationand/or forecasting of the traffic volume associated with the respectivebase station. Once a certain UL/DL resource demand is associated witheach base station, different centralized or distributed methods may beapplied to select the optimal UL/DL configuration mode, either toprovide an initial solution part of a configuration phase or as a(dynamic) adjustment to an existing configuration.

In this regard it is important to note that the UL-DL ratio not onlydepends on the data/resource demand, but also on channel condition andtherefore the UL-DL ratio may vary depending on location andgeographical characteristics.

In order to enable dynamic managing of the UL/DL configuration mode andto allow for fast and effective adaptations a monitoring process may beimplemented that keeps track of the evolving traffic demands to identifymajor alternations or QoS degradation. According to a preferredembodiment a monitoring mechanism is provided in which the base stationsmonitor traffic load in the UL and/or in the DL for locations withintheir respective coverage areas. This monitoring mechanism may becarried out using UE positioning techniques, e.g., the ones described indocument 3GPP TS. 25.111, Technical Specification Group RAN; LMUperformance specification; UE positioning in UTRAN, (Rel 9), December2009. Upon a major change in traffic, which may be identified accordingto predefined criteria, a different UL/DL configuration mode may becomputed. Oscillations among specific UL/DL configuration modes may beavoided by introducing a hold down timer within each UL/DL configurationmode, which prohibits changes of UL/DL configuration modes forpredefined time durations. This would also reduce the minimumre-computation period.

Advantageously, the UL/DL configuration mode selection and adaptationmay be performed by iterative adjustments. For instance, base stationsmay be allowed to iteratively adjust the UL/DL configuration dependingon the QoS demands and how those demands were fulfilled. This processmay be an iterative feedback-loop by allowing base stations to adjustingtheir UL/DL mode/pattern based on the selection of adjacent basestations. Currently selected UL/DL configuration modes may becommunicated among base stations, in particular via the X2 interface incase of 3GPP, and/or towards an associated OAM (Operation,Administration and Maintenance), in particular via the N-Inf.

With respect to enhancing scalability a clustering step may be providedaccording to which base stations with identical or similar UL/DLresource demands and/or geographical properties are grouped to form acluster. The UL/DL configuration mode selection (inclusive of theiterative improvement techniques described above) may then be performedon the level of the clusters rather than considering individual basestations. This could speed up the selection of UL/DL configurationpatterns and may reduce the required overhead. The degree of similaritymay be subject to preadjustment or may be changed dynamically dependingon the current situations.

According to an embodiment varying the UL/DL Configuration Modes may beperformed via the means of interference management. It should be notedthat this approach is not as efficient as the previously describedapproaches, but may prove to be a good solution in certain scenarios,for instance in energy saving scenarios, where the unused spectrum helpsbase stations to switch off certain local components. Similarly to thepreviously described approaches, this scheme also assumes that certainbase stations may have available resources to perform the re-schedulingof certain resources.

At off-peak times the present invention may also contribute to enhancethe energy saving and the efficiency of resource utilization withinTD-LTE by introducing a different UL/DL configuration mode amongneighbor base stations. According to an embodiment an OAM (Operation,Administration and Maintenance) function is provided that is responsiblefor monitoring the network and for identifying off-peak period as wellas keep track of the initial adjustment of the UL/DL configuration mode.It may be provided that during off-peak times a mode is selected with ahigher ratio of UL-subframes in which base stations may turn off theirRF frontend. Consequently, by increasing the share of UL frames andturning off RF frontends the overall energy consumption can besignificantly reduced. This further allows for serving a higher numberof devices with strong UL-demands, such as M2M applications.

The purpose of such UL/DL configuration mode adjustment process is toalign base stations with similar traffic demands by assigning to themthe same UL/DL configuration providing synchronization to avoidinterference, to avoid interference among neighbor base stations withdifferent traffic demands, and to switch off as many unused carriersand/or radio resource as possible to save energy. Thus, this embodimentis similar to the ones described above with an additional parameterbeing energy saving by switching-off unused carriers.

There are several ways how to design and further develop the teaching ofthe present invention in an advantageous way. To this end it is to bereferred to the patent claims subordinate to patent claim 1 on the onehand and to the following explanation of preferred embodiments of theinvention by way of example, illustrated by the figure on the otherhand. In connection with the explanation of the preferred embodiments ofthe invention by the aid of the figure, generally preferred embodimentsand further developments of the teaching will we explained. In thedrawing

FIG. 1 is a schematic view of a cellular communication network thatillustrates the problem of cross-slot interference in TDD systems,

FIG. 2 is a schematic view of a cellular communication network thatillustrates the establishment of UL/DL configuration modes in accordancewith an embodiment of the present invention, and

FIG. 3 is a flow diagram that illustrates an UL/DL configuration modeselection algorithm in accordance with an embodiment of the presentinvention.

It should be noted that even though hereinafter practical details andembodiments of the present invention concentrate on 3GPP LTE scenarios,the same principles apply to any other TDD system.

The lower part of FIG. 2 illustrates a cellular communication networkwhich implements a method for varying the UL/DL Configuration Modes viaTransmission Regulation in accordance with the present invention. Forthe sake of simplicity, only three base stations—eNB A, eNB B, and eNBC—are depicted, even though in real application scenarios typically amuch higher number of base stations will be involved, as will be easilyappreciated by those skilled in the art. The coverage area of therespective cells is indicated by the elliptic curves.

In the scenario illustrated in FIG. 2, eNB A and eNB C are configuredwith different UL/DL configuration modes, illustrated in the upper partof FIG. 2. As described initially in connection with FIG. 1, suchconfiguration may cause severe interference. In the example of FIG. 2,cross-interference would occur either between eNB A and eNB B, which islocated between eNB A and eNB C and which has adjacent or at leastpartly overlapping coverage areas with respect to the base stations, orbetween eNB C and eNB B, depending on the respective UL/DL configurationmode implemented by eNB B. Potentially interfering TDD UL/DLconfiguration modes, i.e. subframes that are potentially subject tointerference, are subframes 3, 4, 6, 7, 8, and 9, since eNB A and eNB Cemploy different access modes in these subframes.

In order to mitigate this problem, in accordance with embodiments of thepresent invention, eNB B either controls the transmission power orcompletely restricts UL and DL transmission for subframes 3, 4, 6, 7, 8,and 9 that may create interference for eNB A and eNB C. In FIG. 2, thelatter solution is indicated by the hatched areas in the respectivesubframes. A main assumption of the proposed solution is the fact thatcertain eNBs may reduce the transmission power or do not transmit orreceive in “silent” sub-frames without significantly affecting the userperformance. This may be accomplished when a significant number of UEsis within the core region of the cell/eNB or when there is no need toutilize all available resources due to lower traffic demands providingthe opportunity for other neighbor eNBs to employ different UL/DLconfigurations.

A specific embodiment of the present invention is illustrated insubframe 6 of FIG. 2, where eNB A has set up a special S subframe,whereas eNB C operates in DL. In this case, eNB B may also set up aspecial S subframe, and it may use the DL part of this frame, which ispossible since it is synchronized with the other neighbor eNB, namelyeNB C in the scenario of FIG. 2, which also uses a DL subframe. In thisregard it is important to note that a DL part of a special frame is likea conventional DL subframe, while the same does not hold for the UL partsince this contains additional synchronization information.

FIG. 3 is a flow diagram that illustrates an algorithm for selecting anoptimal UL/DL configuration mode in accordance with an embodiment of thepresent invention. In a first step, indicated at 300, thetraffic/resource demand is estimated for each eNB. This estimation maybe accomplished by, e.g., relying on UE positioning techniques, and itmay be carried out both for the traffic load in the UL and in the DL. Ina next step, indicated at 302, each eNB is assigned or rated a UL/DLconfiguration mode according to the traffic demand estimated in step300.

Furthermore, indicated at step 304, neighbor eNBs having assigned thesame or a similar UL/DL configuration mode are having similargeographical properties may be grouped to form clusters, which enhancesscalability since improvement techniques for UL/DL configuration modeadaptation can be applied on a cluster basis rather consideringindividual eNBs.

Once the above steps are completed, centralized 306 or distributed 308optimal UL/DL configuration mode selection techniques may be applied.For instance, a centralized approach may start from the highest loadcluster or eNB (in case no clustering step 304 is carried out) and mayselect an optimal UL/DL configuration mode for this cluster/eNB. Next,neighbor clusters/eNBs may be examined, i.e. the next lower loadneighbor cluster or eNB may be determined and the optimal UL/DLconfiguration mode for such cluster or eNB may be selected. This processmay be repeated until an optimal UL/DL configuration mode is selectedfor each cluster/eNB under consideration. In a next step, the selectedUL/DL configuration modes may be adjusted with respect to neighborsuntil all neighbors are examined. Then, the next highest load cluster oreNB may be selected, continuing until all of them are considered.

Distributed approaches may consider only a neighbor clusters or eNBs sothe same process is applicable considering only a neighboring scope.

Embodiments that include the deployment of interference managementmethods UL/DL configuration mode selection may follow the same process.However, instead of selecting the optimal UL/DL configuration mode forclusters and eNBs, they may schedule the neighbor cluster or eNBresources within different spectrum bands.

For energy saving the same steps as described above may be considered,followed by an additional step that aims to switch off the carriers orother eNB component that are not in use.

It should be noted that the power control schemes may also be combinedwith the interference management mechanisms in order to maximize theresource utilization benefits.

Many modifications and other embodiments of the invention set forthherein will come to mind the one skilled in the art to which theinvention pertains having the benefit of the teachings presented in theforegoing description and the associated drawings. Therefore, it is tobe understood that the invention is not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A base station of a cell that operates in TDD (Time Division Duplex)transmission mode, wherein the base station is configured with aplurality of UL/DL configurations which each include a DL subframe, anUL subframe and a special subframe, the UL/DL configurations varybetween the cell and a neighboring cell, and the base station transmitsand receives a UL/DL configuration to and from a base station of theneighboring cell.
 2. A communication method in a base station of a cellthat operates in TDD (Time Division Duplex) transmission mode, whereinthe base station is configured with a plurality of UL/DL configurationswhich each include a DL subframe, an UL subframe and a special subframe,the UL/DL configurations vary between the cell and a neighboring cell,the method comprising: transmitting a UL/DL configuration to a basestation of the neighboring cell; and receiving the UL/DL configurationfrom the base station of the neighboring cell.